1. Field of the Invention
The field of the present invention is electrical devices, and, more particularly, electrical couplers adapted to efficiently transmit high frequency electrical signals.
2. Description of the Prior Art
The design processes for microwave circuits are extremely dependant upon the ability to accurately measure the characteristics of these circuits. To aid in this cause, a great variety of sophisticated measurement equipment has been generated. Unfortunately for the designer working in the microstrip transmission medium, all of this equipment is designed to interface with coaxial or waveguide systems, and a good transition device is necessary to properly utilize this analysis equipment with microstrip lines. The presently available transitions introduce power reflections on the order of 1.8% (VSWR=1.3:1) in the frequency range up to 18 HGz, which can result in impedance measurement errors of 30%. The measurement errors introduced by the transition devices dominate the inaccuracies due to the measurement equipment, thus limiting the overall measurement capability.
One type of prior art coupler shown in U.S. Pat. No. 3,553,607, issued to Lehrfield. The patent shows a connector comprising a mounting plate having a hollow cylindrical section projecting from one surface thereof, threadingly engaging the terminal member of a coaxial line. The hollow interior of the cylindrical portion is fitted with an annular insulating member which supports a conductive pin in a colinear fashion with respect to the center axis of the hollow member. This pin is adapted at one end to couple to the coaxial line center conductor, and its opposite end tapered to form a frusto-conical section, with a short tab extending therefrom to connect to the microstrip conductor. The mounting plate is mounted to the insulating block of the microstrip by screws. A low VSWR for the connector is claimed for frequencies up to 12 HGz. It is clear, however, that the connector does not achieve electromagnetic field matching at the microstrip to coaxial interface; the performance at higher frequencies necessarily is degraded.
Another prior art coupler is shown in U.S. Pat. No. 3,622,915 issued to Davo. The coupler comprises a cylindrical outer conduit with a ramplike conductive member disposed at one end thereof, and an inner conductor having one end adapted to couple to the inner conductor of the coaxial line with a tapered central portion having a slot formed therein to receive a flat strip of conductive material for engaging the conductor of the microstrip. It is claimed that low VSWR is achieved by the connector up to 12 GHz. if the length of the tapered portion of the center conductor exceeds 10 times the difference between its initial and final diameter sizes. However, due to the unusual geometry of the device, it would be extremely difficult to calculate the characteristic impedance of the coupler along its length, and to accurately characterize the device for data interpretation purposes. The 12 Ghz upper limit is relatively low for the high frequency investigations common today.